1. Field of the Invention
The present invention relates to anodes, specifically anodes used in an electrolytic capacitor and a method for producing such anodes.
2. Related Art
Compact, high voltage capacitors are utilized as energy storage reservoirs in many applications, including implantable medical devices. These capacitors are required to have a high energy density since it is desirable to minimize the overall size of the implanted device. This is particularly true of an implantable cardioverter defibrillator (“ICD”), also referred to as an implantable defibrillator or implantable cardiac device, since the high voltage capacitors used to deliver the defibrillation pulse can occupy as much as one third of the ICD volume.
ICDs, such as those disclosed in U.S. Pat. No. 5,131,388, incorporated herein by reference in its entirety, typically use two electrolytic capacitors in series to achieve the desired high voltage for shock delivery. For example, an ICD may utilize two 350 to 400 volt electrolytic capacitors in series to achieve a voltage of 700 to 800 volts.
Electrolytic capacitors are used in ICDs because they have the most nearly ideal properties in terms of size, reliability and ability to withstand relatively high voltage. Conventionally, such electrolytic capacitors include an etched aluminum foil anode, an aluminum foil or film cathode, and an interposed kraft paper or fabric gauze separator impregnated with a solvent-based liquid electrolyte. While aluminum is the preferred metal for the anode plates, other metals such as tantalum, magnesium, titanium, niobium, zirconium and zinc may be used. A typical solvent-based liquid electrolyte may be a mixture of a weak acid and a salt of a weak acid, preferably a salt of the weak acid employed, in a polyhydroxy alcohol solvent. The electrolytic or ion-producing component of the electrolyte is the salt that is dissolved in the solvent. The entire laminate is rolled up into the form of a substantially cylindrical body, or wound roll, that is held together with adhesive tape and is encased, with the aid of suitable insulation, in an aluminum tube or canister. Connections to the anode and the cathode are made via tabs. Alternative flat constructions for aluminum electrolytic capacitors are also known, comprising a planar, layered, stack structure of electrode materials with separators interposed therebetween, such as those disclosed in the above-mentioned U.S. Pat. No. 5,131,388.
Since these capacitors must typically store approximately 30 to 40 joules, their size can be relatively large, and it is difficult to package them in a small implantable device. Currently available ICDs are relatively large (over 36 cm3), generally rectangular devices about 12 to 16 mm thick. A patient who has a device implanted may often be bothered by the presence of a large object in his or her pectoral region. Furthermore, the generally rectangular shape can in some instances lead to pocket erosion at the somewhat curved corners of the device. For the comfort of the patient, it is desirable to make smaller and more rounded ICDs. The size and configuration of the capacitors has been a major stumbling block in achieving this goal.
In ICDs, as in other applications where space is a critical design element, it is desirable to use capacitors with the greatest possible capacitance per unit volume. Since the capacitance of an electrolytic capacitor increases with the surface area of its electrodes, increasing the surface area of an anode foil, for example, results in increased capacitance per unit volume of the electrolytic capacitor. By electrolytically etching the anode foil, an enlargement of a surface area of the foil will occur. Electrolytic capacitors which are manufactured with such etched foils can obtain a given capacity with a smaller volume than an electrolytic capacitor which utilizes a foil with an unetched surface.
In a conventional electrolytic etching process, surface area of the foil is increased by electrochemically removing portions of the foil to create etch tunnels. For example, U.S. Pat. Nos. 4,474,657, 4,518,471, and 4,525,249 to Arora disclose the etching of aluminum electrolytic capacitor foil by passing the foil through an electrolyte bath. The preferred bath contains 3% hydrochloric acid and 1% aluminum as aluminum chloride. The etching is carried out under a direct current (“DC”) and at a temperature of 75° C. U.S. Pat. No. 4,474,657 is limited to the above single step. U.S. Pat. No. 4,518,471 adds a second step where the etched foil is treated in a similar bath with a lower current density and at a temperature of 80-82.5° C. U.S. Pat. No. 4,525,249 adds a different second step, where the etched foil is treated in a bath of 8% nitric acid and 2.6% aluminum as a nitrate, at a temperature of 85° C. Other examples of conventional etching techniques may be found in U.S. Pat. No. 5,715,133 to Harrington et al. and U.S. Pat. No. 4,427,506 to Nguyen et al.
The ideal structure for increasing surface area is a pure tunnel with defined and uniform tunnel diameters. As tunnel density (i.e., the number of tunnels per square centimeter) is increased, a corresponding enlargement of the overall surface area will occur. Larger surface area results in higher overall capacitance. However, traditional etching processes create tunnels of varying and unpredictable length that do not typically extend completely through the anode. These traditional processes also cannot create uniform parallel tunnels. What is needed is a system and method for making anodes having approximately parallel tunnels extending through the anode, increasing capacitance and thereby allowing reduction in overall anode size.